WASHINGTON (Reuters) - Republicans still enjoy a big advantage in enthusiasm about November's US congressional elections, with their party members far more engaged and more likely to vote in a trend that could spell big trouble for Democrats.

President Barack Obama drew millions of first-time voters to the polls and pumped up Democratic turnout in the 2008 election, but those novice voters appear far more likely to sit things out in 2010 with Obama not on the ballot and his approval ratings drifting downward.

That could be a decisive factor in dozens of races nationwide that will decide whether Democrats hold their majorities in the House of Representatives and Senate.

A Reuters/Ipsos opinion poll released on Tuesday found 72 percent of Republicans certain that they will vote in November compared to 49 percent of Democrats.

The findings, similar to those in other national polls, could be crucial to the battle for power in Congress.

"I do think this turnout issue is really going to be the crucial indicator, and the election hangs in the balance on how many of those less-committed Democrats actually turn out again," Ipsos pollster Julia Clark said.

"There may be a sense of complacency, a sense that 'We're done.' That could spell a very, very tough time for Democrats," Clark said.

Turnout is traditionally lower in congressional elections, when there is no presidential race to attract voters, making it crucial for the Democrats to retain the first-time voters drawn to Obama in 2008.

To combat the problem, the Democratic National Committee plans to spend about $30 million to increase turnout. Democratic campaign officials have been developing turnout plans at the state level and for specific congressional districts.

The DNC is targeting 15 million first-time voters who Obama drew to the polls in 2008, many of them young, women, blacks and Hispanics who would be likely to support Democratic candidates in November.

"That is a very real issue that we're focused on," Democratic Representative Chris Van Hollen, who heads the House Democratic campaign committee, told Reuters recently.

The Democratic goal is to increase turnout from the norm by about 10 percent, which the party hopes would be the difference in key battleground states like Ohio, Colorado and Texas.

All 435 House seats and 37 of the 100 Senate seats are up for grabs in November, with Republicans needing to pick up 39 House seats and 10 Senate seats to recapture control.

Democrats point to their win in a May special election in Pennsylvania as a model for November. In a blue-collar typically Democratic House district that backed Republican John McCain for president in 2008, Democrat Mark Critz won with a larger-than-expected turnout from party voters.