Billionaire Tom Steyer is forging ahead with a campaign to attack Rep. Stephen Lynch (D-Mass.) over his support for the Keystone XL oil sands pipeline — despite Rep. Edward MarkeyEdward (Ed) John MarkeyDems lambaste Trump’s ‘outrageous’ EPA chemical safety pick Dems plan to make gun control an issue in Nevada Dem senator pitches ideas for gun control after shooting MORE's (D-Mass.) request that he stay out of the Massachusetts special Senate election race. 

"While we have have great respect for Edward MarkeyEdward (Ed) John MarkeyDems lambaste Trump’s ‘outrageous’ EPA chemical safety pick Dems plan to make gun control an issue in Nevada Dem senator pitches ideas for gun control after shooting MORE and know he is a real Democrat, the issue in this race is whether Steve Lynch is running to be the senator from Canada fighting for increasing the wealth of a foreign oil company or a senator from Massachusetts who will stand for the common good of the Commonwealth," said Chris Lehane, a spokesman for Steyer.

Lynch and Markey are competing to win the Democratic primary for the Senate seat vacated by Sen. John KerryJohn Forbes KerryFor the sake of national security, Trump must honor the Iran deal Bernie Sanders’s 1960s worldview makes bad foreign policy DiCaprio: History will ‘vilify’ Trump for not fighting climate change MORE (D-Mass.) when he became secretary of State. 

Steyer is joining a group of Massachusetts environmental activists to target Lynch over the proposed Keystone XL pipeline, which would ship oil from Alberta's oil sands to refineries on the Gulf Coast of Texas. President Obama and the State Department is currently weighing whether to approve the pipeline's construction. 

Steyer's campaign entails a group of RV trucks with digital billboards that feature a video of Lynch morphing into former President George W. Bush and back again. It highlights Lynch's anti-abortion-rights position, his vote against Obama's healthcare reform law and his support for construction of the Keystone pipeline.

The RVs will be driving through the densest Democratic voting precincts in Massachusetts over the next three days, including those with the highest percentage of voter turnout for President Obama in 2012.

Lynch's support for Keystone is the motivation for Steyer's attack. The California billionaire has spent more than $35 million of his own funds to back progressive ballot initiatives in his state.

Earlier this week, Steyer threatened to spend an unspecified amount in an educational campaign against Lynch if, by noon Friday, he did not denounce the pipeline.

Both Lynch and Markey have signed a pledge to keep outside money out of the race, but interest groups have found loopholes to the pledge, as it only applies to on-air or direct mail attacks. Steyer's roving advertisements do not seem to be covered by the pledge.

Markey's campaign manager Sarah Benzing earlier this week said that the congressman "categorically rejects" any expenditure that would violate the pledge.

Lynch, in a Boston Globe op-ed published Friday morning, accused Steyer of trying to "shove" voters to the left. He characterized Steyer's threat as contrary to the pledge signed that he and Markey signed. 

He called for Markey to "repudiate any and all out-of-state billionaires who attempt to influence this election," and said he would not change his position on the pipeline.

Markey did just that. He called for Steyer to stand down in a release issued Friday, shortly after Lynch's op-ed was published.

"As I stated when I first learned about Tom Steyer's demands on Monday, these kinds of tactics have no place in our political discourse and should be repudiated. Mr. Steyer should immediately withdraw his threats and ultimatum, and stay out of this Senate race. This campaign should be about the people of this state, and I remain committed to giving Massachusetts voters a debate about the issues they care about most," he said in a statement.

Despite Markey's statement, Steyer is moving forward with his efforts, and could spend millions. That level of spending could have an outsized impact on a special election, with a shortened time span and expected low turnout.

But the effort could also backfire, if Markey becomes associated with outside spending and special interest involvement in elections.

Though Lynch hasn't overtly tied Markey to Steyer's attacks, Lynch is now fundraising off his ultimatum. Lynch said in an email to supporters that he's "not about to let [Steyer] shove me around."

"I’ve fought for working families my entire life — and will continue to support them the U.S. Senate. Please donate now to help me fight back," he says in the email, asking for supporters to donate.

It's a populist pitch that's similar to the one used to great effect by Sen. Elizabeth WarrenElizabeth Ann WarrenOvernight Finance: Lawmakers grill Equifax chief over hack | Wells Fargo CEO defends bank's progress | Trump jokes Puerto Rico threw budget 'out of whack' | Mortgage tax fight tests industry clout Michelle Obama is exactly who the Democrats need to win big in 2020 Wells Fargo chief defends bank's progress in tense Senate hearing MORE (D-Mass.) in her 2012 race. She positioned herself as a crusader standing up for the rights of average Americans against special interests, and raised millions on emails similar to Lynch's.

--This post was updated at 1:03 p.m.