It is a peculiar, but unmistakable, phenomenon: As Barack Obama’s presidency heads into its twilight, the rage of the Republican establishment toward him is growing louder, angrier and more destructive.

Republican lawmakers in Washington and around the country have been focused on blocking Mr. Obama’s agenda and denigrating him personally since the day he took office in 2009. But even against that backdrop, and even by the dismal standards of political discourse today, the tone of the current attacks is disturbing. So is their evident intent — to undermine not just Mr. Obama’s policies, but his very legitimacy as president.

It is a line of attack that echoes Republicans’ earlier questioning of Mr. Obama’s American citizenship. Those attacks were blatantly racist in their message — reminding people that Mr. Obama was black, suggesting he was African, and planting the equally false idea that he was secretly Muslim. The current offensive is slightly more subtle, but it is impossible to dismiss the notion that race plays a role in it.

Perhaps the most outrageous example of the attack on the president’s legitimacy was a letter signed by 47 Republican senators to the leadership of Iran saying Mr. Obama had no authority to conclude negotiations over Iran’s nuclear weapons program. Try to imagine the outrage from Republicans if a similar group of Democrats had written to the Kremlin in 1986 telling Mikhail Gorbachev that President Ronald Reagan did not have the authority to negotiate a nuclear arms deal at the Reykjavik summit meeting that winter.