Sen. Mark Pryor (D-Ark.) (AP Photo/Danny Johnston)

A new round of polling in four states central to the battle for the Senate majority shows Sen. Mark Pryor (D-Ark.) with a clear lead over Rep. Tom Cotton (R) and a trio of other incumbents in competitive contests.

Pryor leads Cotton 46 percent to 36 percent among registered Arkansas voters in the New York Times Upshot/Kaiser Family Foundation poll. It's a somewhat surprising finding considering that most other recent polling has shown a neck-and-neck race. The new data come at a time when Democrats are expressing increased optimism about Pryor, who earlier this cycle appeared to be the most endangered senator facing reelection.

The good news for Republicans in the polling is that President Obama's image continues to be a potential liability for Democrats. And in all of the races but Kentucky, supporters of the Republican candidate say they are more likely to vote. Obama's approval rating is in the lows 30s in Arkansas and Kentucky. It's in the low 40s in North Carolina and Louisiana.

In North Carolina, Sen. Kay Hagan (D) (42 percent) is running about even with likely GOP nominee and state House Speaker Thom Tillis (40 percent). Tillis faces a contested Republican primary. If he does not clear the 40 percent mark next month, he will have to face a runoff.

Kentucky is one of Democrats' two best pickup opportunities. There, Senate Minority Leader Mitch McConnell (R) has a real race on his hands. The poll shows McConnell (44 percent) running about even with Secretary of State Alison Lundergan Grimes (D) (43 percent).

In Louisiana, Sen. Mary Landrieu (D) leads Republican Rep. Bill Cassidy 42 percent to 18 percent. But that puts her on pace for a December runoff. Landrieu would have to clear the 50 percent mark to avoid one.

Republicans need to pick up six seats to win back the Senate majority. The polls were conducted April 8-15.