Their base will applaud and vote for them again if they block a deal while everyone one who did not vote for them will still not vote for them (they break-even).
One hint that the government might not block the deal: The two markets combined their clearing operations three years ago without interference from regulators.
U-turn in America's determination to block it from any Caspian pipeline deal: Iran aspires, not unreasonably, to be one of the principal corridors bringing Caspian oil and gas to market.