A bipartisan survey released Thursday -- as well as a handful of Democratic internal polls -- show that the Indiana Senate race between state Treasurer Richard Mourdock (R) and Rep. Joe Donnelly (D) is very close and, as a result,  we are moving the contest from “lean Republican” to “tossup.”

Howey Politics Indiana/DePauw poll released Tuesday shows the race is even -- with Donnelly at 40 percent and Mourdock at 38 percent. (The Howey survey was conducted by pollsters who know Indiana well -- Democratic pollster Fred Yang and Republican pollster Christine Matthews.) Recent Democratic polls from Donnelly’s campaign and a super PAC supporting him tell a similar story.

Meanwhile, Republicans haven’t leaked polls in response to the Democratic releases, suggesting that what they are seeing may not be all that different from what Democrats are touting.

Our national Senate map is below with Indiana now in the "tossup" category. On the map, yellow states are “tossup” races, the light blue and red ones are “lean” Democratic and Republican contests, and the dark blue and red states are “solid” Democratic and Republican races.

With the map as it stands now, if Democrats win everything that is blue, and Republicans win everything red, and the GOP holds the three yellow states it currently controls, it must win the remaining four "tossup" races to win control of the Senate, if President Obama wins reelection.

The fundamentals of the Indiana race still favor Mourdock. Indiana has been trending Republican ever since Obama won the state by about a point in 2008. And Mitt Romney is expected to carry the state in November. Romney has already campaigned with Mourdock, as has GOP vice presidential nominee Paul Ryan.

But a source of Mourdock’s strength in the Republican primary looks like an emerging vulnerability in the general election. He ran to the right of Sen. Dick Lugar, pitching himself as the conservative antidote to the longtime senator. Along the way, he adopted an unapologetic posture to virtually every major issue.

"I hope to build a conservative majority so bipartisanship becomes Democrats joining Republicans to roll back the size of government, reduce the bureaucracy and get America moving again," Mourdock said on CNN a day after he defeated Lugar in May.

In the general election, Democrats have been using Mourdock’s rigid rhetoric to cast him as a hard line partisan, in the hopes of alienating him from the independent voters who for years remained loyal to Lugar.

The AP published a story Thursday detailing Mourdock’s effort to move to the center, which has been a bumpy road. For Mourdock, it doesn't help that Lugar said last week that he won’t campaign with him. Even though he lost the primary, the senator retains considerable good will among the moderate voters Mourdock so desperately needs to win.

Republicans have a straightforward message in this race: A vote for Donnelly is a vote for President Obama and his policies. Given the partisan tilt of the state, that’s a refrain that should have legs down the stretch. But that doesn't absolve Mourdock of the responsibility of building more momentum. He’ll have to convince the electorate that he is preferable to the alternate choice the GOP has set up. So far, he does not appear to have sealed the deal.