Labour has sought to stave off a threatened rout in Scotland at the hands of the Scottish National party by insisting there can be no post-election deals with the nationalists. The main UK opposition party argues that every vote for the SNP risks continued Conservative rule.
In what is expected to be his last speech before Thursday's election, former Labour leader and retiring Kirkcaldy MP Gordon Brown said the party could not "on principle" make any deal with the SNP.
"We cannot have a deal, or a compromise, or a tie-in with a party that doesn't share the principles of solidarity," Mr Brown said during a Glasgow campaign appearance with Scottish Labour leader Jim Murphy.
Mr Brown's sharpened rhetoric on the SNP is intended to shore up core support for Labour, which polls suggest could on Thursday lose all but a handful of the 41 Scottish seats it won in 2010.
However his comments, echoed by Mr Murphy, could complicate a Labour effort to run a minority government if the SNP has a House of Commons swing vote after Thursday's election. Nationalist leaders say they would be able to exert influence over any legislation requiring their support.
Among many opponents of Scottish independence, the prospect of an SNP landslide and its resulting Westminster influence has prompted deep concern. Some have called for tactical voting in favour of the strongest pro-union candidate in each constituency.
Analysts say such voting could save some union party seats but is very unlikely to have a decisive impact in most Scottish constituencies. The main UK parties have shrugged off calls to stand aside in seats that they cannot win.
Labour has been sending voters an eve-of-poll leaflet that calls the election "a two-horse race" between Labour and the Conservatives. "At this election a vote for anyone but Labour risks another Tory government," the leaflet said.
Andrew Skinner of United Against Separation, a pro-tactical voting group, said such leaflets were an unwelcome return to "party politics" in the face of the threat to the UK posed by the SNP.
"It's pretty disappointing, but what can you do? We understand that's what the parties have to say," said Mr Skinner, a longstanding Labour supporter.
Analysts say voters appear to have largely discounted Labour's warnings that voting nationalist will help the Conservatives, in part because of repeated SNP promises to vote to stop any Tory government getting off the ground.
Nicola Sturgeon, SNP leader and Scottish first minister, said on Tuesday that Scotland had never in her lifetime voted Conservative but had been under a Tory UK government more than half the time.
"With more anti-Tory MPs than Tory MPs elected in May, we can lock David Cameron out of Downing Street," Ms Sturgeon said.
The SNP has seized on comments by Ed Miliband, UK Labour leader, suggesting that he would rather not be prime minister than do deals with the SNP.
Ms Sturgeon has also tried to counter doubts about whether it would be legitimate for Labour to form a minority government if it was not the largest party in the House of Commons.
"The test that must be applied is whether a government can build a majority and win support that reflects the whole of the UK," Ms Sturgeon said on Monday.
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