WASHINGTON (AP) — Veteran Democratic Sen. Mark Pryor of Arkansas lost his seat Tuesday to a freshman House Republican, putting the GOP a step closer to its goal of controlling the Senate for the first time in eight years.

Rep. Tom Cotton's win brightened an already happy night for Sen. Mitch McConnell of Kentucky, who won a sixth term of his own and was poised to become the Senate majority leader if his party could gain six new seats overall.

Cotton, an Iraq combat veteran and Harvard Law School graduate, joined virtually every other Republican nationwide in relentlessly linking his opponent to President Barack Obama, whose popularity has sagged.

Cotton claimed one of the six pickups the GOP needs, and Rep. Shelley Moore Capito of West Virginia grabbed another. As expected, she easily won the seat of retiring Democratic Sen. Jay Rockefeller.

As Republicans awaited results elsewhere, they celebrated Sen. Tim Scott 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.

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.

McConnell's allies taunted Grimes for refusing to say whether she had voted for Obama.

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

Democrats privately said they hoped to limit their net Senate losses to five seats, which would barely keep them in control. But even that would require them to win several races Tuesday where they were struggling. Potential runoffs in Louisiana and Georgia, and possible slow vote counts in Alaska, could leave matters unclear for some time.

The GOP seemed certain to pick up three seats where Democratic senators are retiring: in South Dakota and Montana, along with West Virginia.

Republicans felt optimistic about ousting Sen. Mary Landrieu in Louisiana — although a Dec. 6 runoff seemed likely. First-term Democratic Sen. Mark Begich of Alaska was another top target.

Victories in those six states would give Republicans the Senate majority, provided they don't lose any seats they now hold. Their biggest worries in that regard were in Georgia and Kansas.

In Georgia, where GOP Sen. Saxby Chambliss is retiring, Democrat Michelle Nunn and Republican David Perdue were locked in a tight battle.

In Kansas, 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 the GOP's chances.

Elsewhere, contests for Democratic-held seats in three closely divided states could prove crucial.

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.

In Colorado, first-term Democratic Sen. Mark Udall faced a strong challenge from GOP Rep. Cory Gardner. In New Hampshire, Democrats said they believed Sen. Jeanne Shaheen could hold off Republican Scott Brown, a former senator from Massachusetts.

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.

Barring a GOP wave, it's possible that control of the Senate won't be known for days or even weeks.

Slow vote counts in Alaska could make Republican Dan Sullivan's challenge against Begich too close to call for a while. Runoffs were possible in Georgia as well as Louisiana.

If a runoff — or a vote recount in any closely contested state — will determine which party controls the Senate, the spending and politicking will be extraordinary.

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.

Obama could veto legislation passed by a Republican-controlled Congress. And Senate Democrats, if relegated to the minority, could use the filibuster to thwart scores of GOP initiatives, just as Republicans have done to the Democrats for years.

A new Republican Senate majority could be short-lived. The 2016 Senate election map heavily favors Democrats.