By Ros Krasny
BOSTON (Reuters) - Two new surveys show Democrat Elizabeth Warren ahead of incumbent Republican Scott Brown in the race for the U.S. Senate in Massachusetts, one of the year's most costly and closely watched Congressional contests.
Regaining the seat - which Brown won by an upset in 2010 in a special election following the death of Democrat Edward Kennedy - would help the Democrats retain their slim majority in the Senate.
In both surveys, pollsters said Warren had picked up support because Democratic voters seem highly motivated ahead of the November election in a state that President Barack Obama is expected to win by a wide margin.
Warren was leading Brown by six percentage points, or 50 percent to 44 percent, in the Western New England University Polling Institute's survey conducted for the Springfield Republican newspaper.
The survey of 444 likely voters was conducted from September 6 to September 13, starting a day after Warren, a Harvard University law professor and a former official in the Obama administration, gave a nationally televised speech at the Democratic convention.
The poll had a margin of error of plus or minus 4.6 percent. Among a larger sample of 545 registered voters, Warren led Brown by 12 points. In the institute's previous survey taken in late May, Warren led Brown by two points.
Separately, Public Policy Polling, a Democratic-leaning group, showed Warren leading by 45 percent to 43 percent among 876 likely voters.
This poll was taken from Sept 13 to Sept 16 and had a margin of error of plus or minus 3.3 percentage points. In August, PPP showed Brown leading Warren by five points.
PPP director Tom Jensen said that even though Brown was popular among many Democrats, Massachusetts voters in the survey, by a 17-point margin, want Democrats to maintain control of the Senate after the November 6 election.
"More and more Democrats who may like Brown are shifting to Warren because they don't like the prospect of a GOP-controlled Senate," Jensen said.
Brown and Warren both have multi-million-dollar campaign war-chests and have been running television ads across Massachusetts for months.
Brown's campaign has emphasized his credentials as centrist and pro-choice in a bid to win the votes of women, independents and moderate Democrats.
One advertisement highlights Brown's role in authoring a bill to ban insider trading by members of Congress. It shows Obama telling the Republican "good job" at a bill-signing ceremony. Other advertisements have featured local Democratic lawmakers praising Brown.
Tim Vercellotti, professor of political science at Western New England University, said voters who identified themselves as Democrats seemed highly motivated.
"Democrats are more fired up than independent voters at the moment," Vercellotti said. "If that persists, that could pose a problem for Brown, who will need a strong turnout of independent voters to win re-election."
Searching for a decisive edge, Brown and Warren will hold the first of four planned televised debates on Thursday.
Democrats have a 51-47 advantage over Republicans in the 100-seat Senate, with two independents. However, they are defending more than 20 seats against Republican challengers in November, while Republicans are defending only about half that many.
(Reporting By Ros Krasny; Editing by David Brunnstrom)