The SNP is being asked to get behind Labour's plan to reintroduce the 50p tax rate if elected to form the next UK government.
Shadow Scottish Secretary Margaret Curran, a Labour MP, said the move would affect about 14,000, or 0.5%, of taxpayers in Scotland.
"We believe it is fair that in tough economic times, those with the broadest shoulders should bear a little more of the burden for reducing the deficit," she wrote in a latter to Finance Secretary John Swinney, an SNP MSP.
"Over the weekend, SNP representatives appeared to distance themselves from Labour's proposals. It is not clear to me if we can count on the SNP's support.
"I would welcome some clarity from you about whether your party supports the reintroduction of the 50p tax rate and if you will join us in calling for it across the UK."
The tax rate was abolished by the Conservative-Liberal Democrat coalition at Westminster.
If reintroduced by Labour, it would affect those earning more than £150,000 a year.
Ms Curran said the SNP needs to come "off the fence" and pick a side.
Finance Secretary John Swinney said: "We were opposed to the 50p top rate of tax being scrapped in the first place, as it was unfair at a time of recession, and if there is a vote on it now we will vote for its restoration.
"But Labour have no credibility on this issue after Ed Balls and the vast majority of Labour MPs failed to turn up when the SNP proposed a vote against scrapping the 50p rate in 2012.
"Under devolution, we do not have the powers to address this issue, and that would still be the case with the limited tax powers of the Scotland Act.
"In an independent Scotland, we will take a decision on this based on the economic circumstances we inherit at the time, and we will also take forward key policies to tackle inequality, such as scrapping the bedroom tax and ensuring a fair minimum wage that rises at the very least in line with inflation."