Rick Santorum dropped out of the race for the GOP presidential nomination Tuesday, effectively making Mitt Romney the Republican candidate who'll face President Barack Obama. Following are key events and milestones in Santorum's run for the Republican bid.

28 Oct. 1, 2009

Santorum delivers speech at the University of Dubuque on the economy, fueling speculation that he might run for president in 2012.

27 Jan. 15, 2010

Santorum messages supporters of his political action committee, saying, "I'm convinced that conservatives need a candidate who will not only stand up for our views, but who can articulate a conservative vision for our country's future. ... And right now, I just don't see anyone stepping up to the plate. I have no great burning desire to be president, but I have a burning desire to have a different president of the United States."

26 Feb. 20, 2010

Santorum addresses CPAC

FILE - In this Feb. 20, 2010, file photo former Sen. Rick Santorum, R-Pa., addresses the Conservative Political Action Conference (CPAC) in Washington. (AP Photo/Jose Luis Magana, File)

25 April 10, 2010

Santorum speaks to GOP in New Orleans

Former Sen. Rick Santorum, R-Pa., speaks at the Southern Republican Leadership Conference in New Orleans, Saturday, April 10, 2010. (AP Photo/Gerald Herbert)

24 Jan. 30, 2011

Santorum hires key Iowa advisers

In this Dec. 7, 2010, photo, former Pennsylvania Sen. Rick Santorum is interviewed by The Associated Press in Washington. (AP Photo/Manuel Balce Ceneta)

23 April 14, 2011

Santorum forms presidential fund-raising committee, presidential exploratory committee

Former Pennsylvania Sen. Rick Santorum speaks to a crowd at a Tea Party rally at the Statehouse, Friday, April 15, 2011 in Concord, N.H. (AP Photo/Jim Cole)

22 June 6, 2011

Santorum announces candidacy for president of the United States

Former Pennsylvania Sen. Rick Santorum announces he is entering the Republican presidential race, Monday, June 6, 2011, on the steps of the courthouse in Somerset, Pa. (AP Photo/Gene J. Puskar)

21 June 13, 2011

Santorum debates in New Hampshire

From left, former Pennsylvania Sen. Rick Santorum, Rep. Michele Bachmann, R-Minn., former House Speaker Newt Gingrich, former Massachusetts Gov. Mitt Romney, Rep. Ron Paul, R-Texas, former Minnesota Gov. Tim Pawlenty and businessman Herman Cain stand on stage before first New Hampshire Republican presidential debate at St. Anselm College in Manchester, N.H., Monday, June 13, 2011. (AP Photo/Jim Cole)

20 July 5, 2011

Santorum kicks off 'Courage to Fight for American Jobs' tour

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum holds a television interview at his campaign headquarters in Manchester, N.H., Monday, July 18, 2011. (AP Photo/Stephan Savoia)

19 July 29, 2011

Santorum speaks at Denver summit

Former Pennsylvania Sen. Rick Santorum talks to the media prior to speaking at the Western Conservative Summit in Denver on Friday, July 29, 2011.

18 Sept. 23, 2011

Santorum speaks at CPAC

Republican presidential candidate Rick Santorum addresses the Conservative Political Action Conference (CPAC) at the Orange County Convention Center in Orlando, Fla., Friday, Sept. 23, 2011. (AP Photo/Joe Burbank, Pool)

17 Sept. 24, 2011

Santorum campaigns in Florida

Republican presidential candidate former Pennslyvania Sen. Rick Santorum, speaks to delegates before a straw poll during a Florida Republican Party Presidency 5 Convention Saturday, Sept. 24, 2011, in Orlando, Fla. (AP Photo/John Raoux)

16 Oct. 20, 2011

Santorum files for New Hampshire ballot

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum gestures as he files paperwork to be on the ballot in the presidential primary, Thursday, Oct. 20, 2011, at the New Hampshire Statehouse in Concord, N.H. (AP Photo/Elise Amendola)

15 Dec. 28, 2011

Santorum campaigns in Iowa

Republican presidential candidate former Pennsylvania Sen. Rick Santorum gestures as he waits to speak to local residents during a campaign stop at USA Furniture and Bedding, Wednesday, Dec. 28, 2011, in Dubuque, Iowa. (AP Photo/Charlie Neibergall)

14 January 2012

More than two weeks after Iowa Caucuses, Santorum declared official winner, beating Romney by 34 votes

Republican presidential candidate former Pennsylvania Sen. Rick Santorum, center, waves to supporters during his speech at his Iowa caucus victory party Tuesday, Jan. 3, 2012, in Johnston, Iowa. (AP Photo/Charlie Riedel)

13 Jan. 8, 2012

Santorum debates in New Hampshire

Former Pennsylvania Sen. Rick Santorum, left, and former Massachusetts Gov. Mitt Romney spar during a Republican presidential candidate debate at the Capitol Center for the Arts in Concord, N.H., Sunday, Jan. 8, 2012. (AP Photo/Charles Krupa)

12 Jan. 16, 2012

Santorum debates in South Carolina

Republican presidential candidates, from left to right: Texas Gov. Rick Perry; former Pennsylvania Sen. Rick Santorum; former Massachusetts Gov. Mitt Romney; former House Speaker Newt Gingrich; and Rep. Ron Paul, R-Texas, participate in the South Carolina Republican presidential candidate debate in Myrtle Beach, S.C., Monday, Jan. 16, 2012. (AP Photo/Charles Dharapak, Pool)

11 Feb. 7, 2012

Santorum wins Colorado and Minnesota, as well as non-binding Missouri primary

Children play in front of a stage before the start of a primary night watch party for Republican presidential candidate, former Pennsylvania Sen. Rick Santorum Tuesday, Feb. 7, 2012, in St. Charles, Mo. (AP Photo/Jeff Roberson)

10 Feb. 10, 2012

Santorum addresses CPAC

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum, flanked by his wife Karen, right, and daughter Elizabeth, addresses the Conservative Political Action Conference (CPAC) in Washington, Friday, Feb. 10, 2012. He is joined by his wife Karen at right. (AP Photo/J. Scott Applewhite)

9 Feb. 22, 2012

Obama's healthcare plan debated

FILE - In this Feb. 22, 2012, file photo Republican presidential candidates, former Massachusetts Gov. Mitt Romney, right, and former Pennsylvania Sen. Rick Santorum, argue a point during a Republican presidential debate in Mesa, Ariz, during which Santorum confronted Romney about "Obamacare." All four GOP candidates now promise to repeal it, with Santorum calling it “the death knell for freedom.” Its model was a Massachusetts law signed by frontrunner Romney, when he was governor. (AP Photo/Jae C. Hong, File)

8 March 6, 2012

Santorum wins Tennessee, Oklahoma and North Dakota, loses close race to Romney in Ohio

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum speaks to supporters at an election night party at Steubenville High School in Steubenville, Ohio, Tuesday, March 6, 2012. (AP Photo/Gene J. Puskar)

7 March 10, 2012

Santorum wins Kansas caucus

Kansas Gov. Sam Brownback talks to election workers from the state Republican Party after checking in at a presidential caucus, Saturday, March 10, 2012, in Topeka, Kan. Brownback wouldn't say which candidate he supported. Rick Santorum overwhelmingly won the Kansas Republican presidential caucuses. GOP front-runner Mitt Romney did not campaign in Kansas.(AP Photo/John Hanna)

6 March 13, 2012

Santorum wins Alabama and Mississippi primaries

In this Monday, March 12, 2012, photo, Republican presidential candidate, former Pennsylvania Sen. Rick Santorum campaign in Montgomery, Ala. Santorum won the Republican presidential primary Tuesday, March 13, in Alabama (AP Photo/Eric Gay)

5 March 22, 2012

Santorum uses Etch A Sketch prop

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum fiddles with an Etch A Sketch as he speaks to USAA employees during a campaign stop, Thursday, March 22, 2012, in San Antonio. Santorum used the toy to refer to remarks made by a staff member working for Republican presidential candidate, former Massachusetts Gov. Mitt Romney. (AP Photo/Eric Gay)

4 March 24, 2012

Santorum wins Louisiana Primary

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum, right, is joined by his daughter Sarah Maria during a news conference in Green Bay, Wis., Saturday, March 24, 2012. Santorum won the Louisiana Republican presidential primary Saturday, beating front-runner Mitt Romney in the race to challenge President Barack Obama. (AP Photo/Jae C. Hong)

3 April 2, 2012

Santorum campaigns in Wisconsin

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum signs his autograph on a cheese head hat at Simon's Specialty Cheese in Appleton, Wis., Monday, April 2, 2012. (AP Photo/Jae C. Hong)

2 April 6, 2012

Santorum campaign halts while his daughter is hospitalized.

FILE - In this June 6, 2011 file photo, Republican presidential candidate, former Pennsylvania Sen. Rick Santorum holds his daughter Isabella in Somerset. The ill daughter of Republican presidential candidate Rick Santorum has been hospitalized. Santorum's campaign said Friday that Bella had been taken to an undisclosed hospital. The 3-year-old has the rare genetic condition Trisomy 18 and was hospitalized earlier this year with pneumonia. Santorum's campaign did not say why she was hospitalized. (AP Photo/Gene J. Puskar, File)

1 April 10, 2012

Santorum suspends campaign in Gettysburg, Pa.

Republican presidential candidate, former Pennsylvania Sen. Rick Santorum turns to his wife Karen, left, after announcing he is suspending his candidacy effective today in Gettysburg, Pa., Tuesday, April 10, 2012. (AP Photo/Gene J. Puskar)