White House Chief of Staff Rahm Emanuel, a former chairman of the Democratic Congressional Campaign Committee, “was well aware of the political ramifications surrounding this selection when this plan was hatched,” the memo said. “The party boss in the West Wing saw a political opportunity and he seized on it.”

Democrats dominate the state’s congressional delegation, holding both Senate seats and 26 of 29 House seats after picking up three in last year’s election.

Republicans lost upstate districts to Democrats Eric Massa of Corning and Dan Maffei of DeWitt, as well as a New York City district covering Staten Island and part of Brooklyn, where Democrat Mike McMahon won an open seat.

If the Senate confirms McHugh, the only New York Republicans serving in Congress will be Rep. Peter King of Long Island and freshman Rep. Chris Lee from the Buffalo suburb of Clarence.

Early handicapping by the nonpartisan Cook Political Report is that the race for McHugh’s seat will be a tossup.

Although McHugh’s 23rd District has a long history of being represented by Republicans, Democrats see an opportunity. Obama picked up 52 percent of the vote in the district in last year’s presidential election.

That’s slightly better than he did in the 20th District, where Democrat Scott Murphy won a special election earlier this year. The win kept Kirsten Gillibrand’s former House seat in the Democratic column after Gillibrand was appointed to the Senate by Gov. David Paterson.

Murphy’s close victory over Republican state Assemblyman Jim Tedisco was not considered an upset because a Democrat had won the seat in 2006 and 2008.

That hasn’t been the case in the 23rd District, where McHugh, a former state senator, has represented the state’s North Country since 1992. The district covers all of or part of 11 counties, with about half of them bordering Vermont or the Canadian provinces of Ontario and Quebec.

Registered Republicans in the district outnumber registered Democrats by more than 46,000 voters.

“Despite the edge Republicans have in voter registration, it’s at least an even contest,’’ said Ivan Kenneally, an assistant professor of American politics at the Rochester Institute of Technology.

He said the parts of the district along the Canadian and Vermont borders and on the edge of Syracuse have been trending Democratic.

David Wasserman of the Cook Political Report said the Republican-leaning voter registration numbers and Democratic-leaning presidential election results demonstrate the district is competitive.

“It’s unfair to say this district leans one way or another,’’ he said. “It’s really a middle-of-the-road district.’’

On the Democratic side, party leaders are unlikely to choose a state senator such as Darrel J. Aubertine of Cape Vincent or David Valesky of Oneida because it could result in a loss of a Democratic majority in that chamber, where Democrats hold a 32-30 edge.

“There is no leading Democrat,’’ said Rep. Steve Israel, national recruiting chairman for the DCCC. “This is very early in the process. John McHugh won’t be confirmed for four to six weeks.’’

Israel held a telephone conference call Tuesday afternoon with Democratic county leaders who will choose the candidate.

Several Republicans – state Sen. Joseph Griffo of Rome; state Assemblyman Will Barclay of Pulaski; state Assemblywoman Dede Scozzafava of Gouverneur, and Franklin County District Attorney Derek Champagne – said Tuesday they were considering the race.

“It’s been an honor to serve a lot of that area and advocate for that area,” said Griffo.

Another possible candidate is Republican Robert Taub, McHugh’s chief of staff.

The governor won’t set the date for a special election until after the vacancy occurs.