WASHINGTON — Republicans took giant steps toward controlling the Senate on Tuesday, ousting Democratic senators in Colorado and Arkansas while replacing retiring Democrats in three other states.

With several Democratic seats still in play, six-term Sen. Mitch McConnell of Kentucky seemed almost within reach of becoming the head of a new Republican majority and creating new political problems for President Obama. Barring a Republican loss in Kansas, the party needed only one more take-away to claim the majority in the next Congress.

Republicans claimed a huge victory in Colorado, where Rep. Cory Gardner ousted first-term Democrat Mark Udall. The win was notable because Obama had carried Colorado twice, unlike the other states where Republicans were making their biggest gains.

Udall portrayed Gardner as a threat to women’s reproductive rights. But Gardner responded with the tactic used by every Republican in a competitive race: relentlessly linking his opponents to a president whose popularity has sagged.

In Arkansas, freshman Rep. Tom Cotton knocked off two-term Democratic Sen. Mark Pryor in a state that has veered sharply Republican since native son Bill Clinton left office.

A DEEPER SHADE OF RED

Republicans also easily replaced retiring Democratic senators in Montana, West Virginia and South Dakota.

That gave them five of the six new seats they needed to claim the Senate majority for the first time in eight years. With their solid House majority, they’d control both congressional chambers for the rest of Obama’s presidency.

A bright spot for Democrats was Sen. Jeanne Shaheen’s victory in New Hampshire over Scott Brown, a former senator from Massachusetts.

McConnell, of Kentucky, won a sixth term of his own, and immediately warned Obama of coming confrontations. “For too long, this administration has tried to tell the American people what’s good for them and then blame somebody else when their policies didn’t work out,” McConnell told cheering fans.

If he becomes majority leader, McConnell, 72, would have substantial powers to decide what legislation reaches the floor for votes, and when.

Cotton, an Iraq combat veteran and Harvard Law School graduate, linked Pryor with Obama in every campaign appearance.

POST-CLINTON CHANGE

Pryor, the last Democrat in Arkansas’ congressional delegation, is the son of a popular former governor and senator. But Arkansas and West Virginia have been trending sharply Republican. Obama lost Arkansas by 24 percentage points in 2012.

In Kentucky, Democrats once had high hopes for challenger Alison Lundergan Grimes, the state’s young secretary of state. But the hill was too steep in a state Obama lost by 23 percentage points in 2012.

REPUBLICANS REPLACE RETIREES

As expected, Republican Rep. Shelley Moore Capito of West Virginia won the seat of retiring Democratic Sen. Jay Rockefeller. Former Gov. Mike Rounds of South Dakota won retiring Democratic Sen. Tim Johnson’s seat. And Republican Rep. Steve Daines will succeed departing Sen. John Walsh in Montana.

In Georgia, where Republican Sen. Saxby Chambliss is retiring, Republican corporate executive David Perdue held off Democrat Michelle Nunn.

In Louisiana, Republicans hoped for a delayed pickup, as GOP Rep. Bill Cassidy forced three-term Democratic Sen. Mary Landrieu into a Dec. 6 runoff.

In Virginia, Republican Ed Gillespie mounted an unexpectedly strong challenge to first-term Democratic Sen. Mark Warner.

In North Carolina, Democratic Sen. Kay Hagan was facing state House speaker Thom Tillis. The race set records for campaign spending, with airwaves drenched in political ads. Obama carried North Carolina in 2008, and lost it in 2012.

IMPACT REMAINS TO BE SEEN

Few campaigns were as feisty and close as Iowa’s, where long-time Democratic Sen. Tom Harkin is retiring. Republican state Sen. Joni Ernst was facing Democratic U.S. Rep. Bruce Braley in a race that featured TV ads about castrating hogs, and a leaked fundraising video from Texas.

First-term Democratic Sen. Mark Begich of Alaska faced Republican Dan Sullivan in a state Obama lost badly.

As Republicans awaited results elsewhere, they celebrated Sen. Tim Scott of South Carolina becoming the first black elected to the Senate from a former Confederate state since Reconstruction. He was appointed to the Senate last year, and won a term of his own Tuesday.

Kansas remained a worry for Republicans. Three-term Republican Sen. Pat Roberts was scrambling to fend off independent candidate Greg Orman, who had persuaded the Democrat to leave the race and help him consolidate anti-Roberts sentiment. Orman hasn’t said which party he will caucus with, however, so a Roberts loss doesn’t automatically endanger Republican chances.

A Republican takeover of the Senate would be huge politically, but its impact on governing is unclear.

Even with Republicans controlling both houses of Congress, many of the dynamics that have fed federal gridlock for years would still be present.