HARRISBURG — Top Republicans have been unable to agree on a new map of Pennsylvania’s 18 congressional districts after weeks of private meetings and, with campaign season already under way, they acknowledge they are still working through differences over politics and geography.

The map must undergo significant changes — the delegation will shrink by one seat in the 2012 election — that give Republicans the opportunity to force two Pittsburgh-area Democrats, Jason Altmire and Mark Critz, to run against each other in one district.

However, the harder problem for Republicans in this once-a-decade task appears to be how to spread sympathetic voters more evenly to strengthen the electoral chances of their 12 incumbent U.S. House members, while attempting to draw a district that favors a 13th Republican.

Candidates may shy away from running for a two-year U.S. House term while the borders of districts remain in limbo, and the time to get a map to Republican Gov. Tom Corbett’s desk for his signature is shrinking: Jan. 24 is the first day for candidates to circulate nomination petitions.

“In a drop-dead scenario, that’s it,” said Senate State Government Committee Chairman Charles McIlhinney, R-Bucks.

So far, no draft of a map has been made available to the public or open for public scrutiny — a top complaint of Democrats who stand to lose one of their seven seats and see more of their most reliable voters packed into fewer districts.

“They’re going to just ram (a map) through as quickly as possible without any opportunity for public comment or anyone to get a good understanding of these districts, which is a disservice to the people in the state,” said state Democratic Party spokeswoman Lindsay Fritchman.

McIlhinney said Friday he hopes to unveil a proposed new map Monday and hold a vote in his committee Tuesday. That would allow the full Senate to send it to the House on Wednesday, the Senate’s last scheduled session day this year. The House is scheduled to hold three more voting session days this year, Dec. 19-21, before both chambers break for the holidays until January.

In any case, that’s a more public process than a decade ago, when senators voted on a map an hour after they first saw it, McIlhinney said.

A spokesman for House Majority Leader Mike Turzai, R-Allegheny, would only say that Republicans are working to ready a map for Monday’s unveiling, while several Republican U.S. House members declined to comment or respond to telephone messages. The last map was approved Jan. 7, 2002, when Republicans controlled Harrisburg.

Thanks to their control of Harrisburg again, Republicans get to draw the new congressional map that is required every decade to reflect population shifts. Because Pennsylvania grew more slowly than the rest of the nation, it will lose a U.S. House seat, dropping from 19 to 18.

One source of tension is how Republican-friendly to make the new district that combines parts of Altmire’s 4th District and Critz’s 12th District.

To ensure both Democrats live in it, it will have to stretch and wind across several southwestern Pennsylvania counties.

One map distributed last week and shown to The Associated Press would pave the way for a Republican challenger to win, but doing so could weaken neighboring Republican incumbents Tim Murphy and Mike Kelly, some Republicans say.