AUGUSTA — The battle for control of the Maine House and Senate is shaping up as the parties put the final touches on their slate of candidates.

Parties have until Monday to submit replacements for candidates who have dropped out since the June 10 primary. But dozens of contests are already being closely watched as Democrats hope to fend off Republicans who are eager to return to power in both chambers.

One of Democrats’ biggest strengths is having more incumbents on the ballot, which they say shows the party’s enthusiasm going into November. Even so, Republicans are being backed by powerful national groups that view Democrats’ majorities as vulnerable and are expected to pump thousands of dollars into the state.

With only a few seats separating Democrats and Republicans in the Senate, the parties are already eyeing the race in Bangor, which is certain to again be among the most expensive and high profile this year.

Both parties see opportunity in the swing district that includes Bangor, where voters tend to vote independent or Democratic, and the conservative town of Hermon, which Republican Gov. Paul LePage won in 2010.

The contest, which attracted more than $450,000 in outside spending two years ago, pits Democratic Sen. Geoff Gratwick against former Bangor Mayor Cary Weston, who considers himself a moderate Republican.

Gratwick and Weston, who served on the Bangor City Council together, said they hope the campaign remains positive and focused on the issues — like health care and jobs — rather than on personal attacks. But with outside groups expected to invest heavily, that could be beyond the candidates’ control.

“Hopefully we can rise above it,” Weston said. “It’s going to be loud and hard to do, but that is our goal.”

Democrats have a long history of liberal legislative control on their side. Before the Republicans flipped both chambers in 2010, Democrats took the majority in the House in every general election since 1974, while Republicans held the Senate outright for only one term since the 1982 elections.

Democrats also have more than double the number of incumbents running in the House and five more sitting senators on the ticket than Republicans. Beyond that, part of their strategy will be tying Republican lawmakers to LePage, whose views they say are out of touch with voters.

“Where (LePage) did things that we think have hurt the state of Maine, we are going to make sure that voters understand that that their legislators voted in lock step with the governor,” said Democratic Party Chairman Ben Grant.

Yet, Republicans only need to pick up three seats — like Gratwick’s — in the 35-member Senate, which now has 19 Democrats, 15 Republicans and one unenrolled. The 151-person House has 89 Democrats, 58 Republicans, two independents and two unenrolled.

That’s drawn the attention of groups like the Republican State Leadership Committee, which has put both chambers on its “Sweet 16 Targets” list and has pledged to assist Maine Republicans in their effort.

Other races being closely watched include those where longtime lawmakers are attempting to make a return, like Democrat Bill Diamond who’s running against Republican Toby Pennels for the Senate in Windham. Democrats are hopeful that Diamond, a former Maine secretary of state and member of both legislative chambers, can pick up the seat that’s being vacated by Republican Gary Plummer.

Meanwhile, Republicans see several opportunities to pick up seats in the House, including in Presque Isle, where Larry Fox is challenging freshman Democratic Rep. Bob Saucier. Republican Party spokesman David Sorensen said Saucier’s party-line votes make him vulnerable in the more conservative northern Maine district.

The divisions within the GOP may have caused infighting within the Maine Republican Party over the last several years. But Sorensen said the wide variety of views is one of the party’s biggest strengths in the legislative races where a moderate can find success in central Maine and a more conservative candidate is better poised to win in the north.

“Overtime there has been increasing tendency of the Democratic caucus to be in lockstep,” he said. “When everyone is voting 90 percent like each other, you’ve got a situation where what may be popular in Portland isn’t necessary popular in Presque Isle.”