Here's What's Happened Since the Last GOP Debate

— -- A lot has happened in the six weeks since the last Republican presidential debate.

Here's what you need to know before the GOP candidates take the stage tonight in Colorado for their third debate.

Walker Drops Out

Once thought to be a leading contender for the Republican nomination, Wisconsin Gov. Scott Walker announced he was suspending his presidential campaign on Sept. 21 – less than a week after the second GOP presidential debate.

Carson Doesn’t Think A Muslim Should Be President

On Sept. 20, Dr. Ben Carson said on NBC’s “Meet The Press” that “I would not advocate that we put a Muslim in charge of this nation, I absolutely would not agree with that.”

Democrats Debate

The first Democratic presidential debate on Oct. 13 kicked off a winning streak for Democratic presidential candidate Hillary Clinton. She made an impression again the following week during the all-day Benghazi Committee hearing.

Joe Says “No”

After months of speculation, Vice President Joe Biden, speaking in the White House Rose Garden, told the world on Oct. 21, “I believe we are out of time – the time necessary to mount a winning campaign.”

Carson Catches Up To Trump

Once leading the pack by a good margin, Donald Trump saw his status as the Republican front-runner begin to fall as Carson’s star began to rise. A NYT/CBS national poll released Oct. 27 showed Carson pulling into the number one spot.

ABC News' Tom Thornton contributed to this production.