Alberta Premier Jason Kenney and the leaders of the three other provincial parties are offering an olive branch to the Trudeau government on C-69, saying they're now prepared to accept the controversial overhaul of Canada's environmental assessment process — as long as the Senate's amendments are part of it.

In a joint letter to Prime Minister Justin Trudeau's point man in the Senate, Peter Harder, the multipartisan group asks the government to accept the changes to Bill C-69 — including amendments long demanded by oil and gas lobbyists — to avoid a constitutional fight over federal-provincial jurisdiction in natural resources.

"While we remain concerned about the overall spirit of Bill C-69, we believe that with the inclusion of all these amendments, that the bill would be acceptable to the interests of Albertans," reads the letter, signed by Kenney, NDP Leader Rachel Notley, Alberta Party Leader Stephen Mandel and David Khan, the leader of the Alberta Liberal Party.

The Senate's energy committee last week passed more than 180 amendments that would, among other things, limit the environment minister's ability to interfere in the regulatory process and stop and start project timelines.

The committee also passed amendments to curb public participation in the project review process to ensure more timely decisions, and backed changes that would solidify the role of the offshore petroleum boards in the approvals process.

"We call upon the entire Senate to likewise respect the deliberations of the standing committee ... and vote in favour of the entirety of this amendment package," the leaders said.

The letter signals a tonal shift for Alberta's leaders. Kenney has long called for the Senate to kill the bill outright, while Notley described the Liberal proposals as a "stampede of stupid."

The letter, obtained by CBC News, says Bill C-69 "in its unamended form would seriously threaten Alberta's exclusive provincial jurisdiction over the regulation of the production of non-renewable natural resources."

The leaders also asked Harder to accept the Senate transport committee's deadlock over C-48, the northern B.C. oil tanker ban bill. The committee's vote on whether to recommend the bill for passage last week ended in a tie, meaning the recommendation failed.

Reached by phone Thursday, Harder told CBC News that he believes the Senate should pass the bill as amended by the committee and send it back to the House of Commons, where the government can decide which amendments it's willing to accept.

That's a change in approach for Harder; his lieutenant, government liaison Sen. Grant Mitchell, pushed back against a number of proposed amendments during the committee process, saying they do not square with the government's goal of dramatically reworking the assessments process.

"I always say I'm not just the government's representative in the Senate. I'm the Senate's representative to the government," Harder said.

Sen. Peter Harder speaks on Parliament Hill. Harder said the Senate should pass the bill as amended and send it back to the Commons where the government can decide on which amendments to Bill C-69 it is willing to accept. (Justin Tang/Canadian Press)

"Where we're at now, after the Senate committee deliberations, is a large number of amendments, some of which are not necessarily in harmony with others. My view is it would be best for us, as a Senate, to act reasonably quickly and send the bill back to the House of Commons for the government to put forward which of the amendments it would agree to. And then that would come back to the Senate."

(After legislation is amended by the Senate, the amended version must also pass the Commons before it becomes law.)

Environmental groups have slammed the Senate's amendments as a cut-and-paste job lifted from material submitted by oil industry lobbyists. In fact, the wording of many of the amendments is identical to what was asked for by energy lobby groups, including the Canadian Association of Petroleum Producers.

Regarding C-48, Harder said Trudeau promised a similar tanker ban during the last federal election and a campaign promise should be respected by the unelected upper house.

"We are a revising chamber, not a defeating chamber," Harder said.

Only days after he was elected premier in Alberta's April election, Kenney appeared before the Senate's energy committee, where he urged senators to make major changes to Bill C-69. He said Ottawa's attempt to rewrite existing assessment legislation, do away with the National Energy Board and bolster Indigenous participation in the approvals process — among many other changes to the natural resources regime — would create uncertainty for an industry that is facing constrained pipeline capacity and cratering commodity prices.

"There is a growing crisis of national unity in Alberta which would be exacerbated by the adoption of this bill and other policies like it," Kenney said. "If this bill proceeds, it will be a message to the people of Alberta that their federal government doesn't care about a devastating period of economic adversity in our province."

Kenney also said he was prepared to launch a constitutional legal challenge against the legislation if it was passed by the Senate as written, saying Ottawa is unfairly intruding on an area of provincial jurisdiction.