WASHINGTON — A moderate Republican senator sought broad bipartisan support Tuesday for a compromise to block gun purchases by some suspected terrorists, a day after the chamber split along party lines to derail far more sweeping proposals. Senate Majority Leader Mitch McConnell, R-Ky., said he would allow a vote on the proposal by Sen. Susan Collins, R-Maine, but stopped short of endorsing the measure itself. The package seemed to face an uphill climb for the 60 votes it would need, thanks to the hurdles of election-year politics and opposition from the National Rifle Association and Gun Owners of America.