Conservative MPs from Saskatchewan are urging Prime Minister Stephen Harper to keep the party's campaign promise on federal equalization payments.

If he doesn't, according to a letter signed by Saskatchewan Conservative caucus chair Brian Fitzpatrick that was leaked to the media, it will cause "no end of political difficulty" in the next election.

In the July 25 letter, Fitzpatrick, who is the MP for Prince Albert, said equalization was the dominant issue discussed at a July 19 meeting of Saskatchewan caucus MPs.

Equalization is the $11-billion federal program under which Ottawa makes payments to so-called "have-not" provinces. Thanks to the recent surge of oil prices, Saskatchewan is considered a "have" province and doesn't qualify for money from Ottawa under the program.

However, the Conservatives campaigned on a promise to change the equalization formula — by returning to a 10-province standard and exempting non-renewable resources such as oil and gas.

During the campaign, Regina MP Tom Lukiwski said those two changes could mean at least $2 billion a year extra for Saskatchewan government coffers. The Conservatives ended up winning 12 out of 14 federal seats in Saskatchewan.

More than half a year later, nothing has changed and Harper has since referred to the equalization promise as more of a preference.

His Saskatchewan MPs say that's not good enough.

"All members present believed that anything less than substantial compliance with our commitment will cause us no end of political difficulty during the next federal election," Fitzpatrick said in the letter that was addressed to both Harper and Finance Minister Jim Flaherty.

Fitzpatrick said he won't comment on a letter he intended to be private.