On Monday, the Republican Party revealed that its new smear campaign against Democratic vice presidential candidate Tim Kaine will double as something else: an all-out assault on the United States Constitution. Kaine, you see, served as a defense attorney before taking public office and defended several extremely unpopular and unsavory clients. Taking aim at this record, the Republican National Committee released an ad attacking Kaine for defending murderers and rapists—“the worst kinds of people.” Throughout the ad, a narrator describes the crimes of a number of Kaine’s clients, then says with disgust at each: “Tim Kaine defended him.”

“Tim Kaine,” the ad concludes: “He has a passion for defending the wrong people. America deserves better.”

The RNC’s spokesman promptly touted the ad, initially endorsing it as a “Willie Horton-Style Attack” on Kaine. But the oddest thing about the spot isn’t that it proudly borrows from one of the darkest, most racist chapters in American electoral history. What’s truly strange about the ad is that it constitutes a straightforward, unapologetic attack on the Sixth Amendment. With it, the Republican Party can never again claim in good faith to be the party of defending the Constitution.

Read the Sixth Amendment, and you’ll quickly spot the Assistance of Counsel Clause: “In all criminal prosecutions,” the amendment states, “the accused shall enjoy the right … to have the Assistance of Counsel for his defense.” As I’ve explained, the right to counsel is such a bedrock principle of American jurisprudence that it predates the Constitution. John Adams famously defended the British soldiers accused of committing the Boston Massacre, arguing that even the most reviled defendants deserved a strong defense. He secured acquittals for most of the soldiers and later wrote that his defense was “one of the most gallant, generous, manly and disinterested Actions of my whole Life, and one of the best Pieces of Service I ever rendered my Country.”

When James Madison proposed a Bill of Rights to shore up the new Constitution’s protection of individual rights, he included a right-to-counsel clause in his earliest drafts. The clause enjoyed nearly universal support at ratifying conventions, since the colonists had suffered under the old English rules barring many defendants from assistance of counsel. Indeed, a majority of colonies already guaranteed the right to an attorney before the Sixth Amendment was even ratified. Moreover, for much of American history, defending vilified clients was seen as an honorable, even noble calling. John Jay and Alexander Hamilton represented universally deplored defendants without apology. So did Abraham Lincoln, who defended several notorious murders. Lincoln, in fact, defended an accused murderer just a year before he was elected president. His client got acquitted.

Today we often debate more modern interpretations of the Assistance of Counsel Clause—when counsel becomes constitutionally ineffective, for instance, or when the state must appoint an attorney for indigent defendants. But no one seriously questions that the Sixth Amendment guarantees all defendants the right to an attorney. And defending loathsome clients is widely considered a high calling, a fulfillment of an attorney’s most basic duties, and a vindication of defendants’ constitutional rights.

The Republican Party, however, now tells a different story. To its national committee, its lobbyist allies, and even its presidential candidate, offering to provide a loathsome client his Sixth Amendment rights is dishonorable and deplorable—proof of a character flaw so great that it disqualifies an attorney from public office.

This line of attack marks an outright rejection of one of America’s most fundamental constitutional values. It also means that the GOP can never again claim the constitutional high ground in any given debate. Both parties propose measures that arguably infringe on some provision of the Constitution. But this smear campaign is categorically different—a repudiation of the Constitution’s explicit text, intent, and history. It is an ugly new chapter in modern American politics. And it could deter politically minded attorneys from defending allegedly vile clients out of a reasonable fear that their defense could sabotage their political future.

Again, Democrats do support their share of plausibly unconstitutional policies. Their attempt to prevent individuals on the terrorist watch list from purchasing firearms, for example, may violate due process. Their push to overturn Citizens United could arguably jeopardize First Amendment rights. Republicans have fiercely condemned both these efforts as attacks on the Constitution. And I happen to agree that the party’s current proposals to both limit Citizens United and institute “no fly, no buy” are terrible ideas of dubious constitutionality.

But even if these measures are unconstitutional, they aren’t attacks on the Constitution itself. They are attacks on modern—indeed, very recent—interpretations of the Constitution. The due process problems with “no fly, no buy” are rooted in the Supreme Court’s 5–4 decision in 2008 holding, for the first time ever, that the Second Amendment protects an individual right to gun ownership. (Because bearing arms is now a “fundamental right,” the argument goes, restrictions on firearm ownership should be subject to stricter due process limitations.) The constitutional problems with bills curbing Citizens United hinge on another 5–4 ruling in 2010 that reversed decades of precedent. Meanwhile, Republicans’ constant complaint that President Obama’s executive actions and regulatory rulemaking violate separation of powers or the 10th Amendment has yet to find support with a majority of the court.

This new offensive is altogether different, and it has been building for several years. In 2014, Senate Republicans viciously attacked Debo Adegbile, Obama’s nominee to the Civil Rights Division of the Justice Department, for defending a murderer, successfully blocking his confirmation. A month later, the Republican Governors Association ran an ad attacking Vincent Sheheen, the Democratic nominee in the South Carolina gubernatorial race, for “personally defending dangerous criminals” as a defense attorney. More recently, the National Republican Senatorial Committee put out an ad slamming Hillary Clinton for defending “an accused child rapist.” The conservative Judicial Crisis Network lobbying group, meanwhile, pre-emptively attacked Obama Supreme Court shortlist-er Jane Kelly for her defense of a child molester. And Republican presidential nominee Donald Trump has said that the right of an accused terrorist to be “represented by an outstanding lawyer” is a “sad situation.” The Republican Party has essentially declared that the performance of an attorney’s Sixth Amendment duties is unforgivably execrable. It might as well have declared that the Sixth Amendment itself is reprehensible. How else to explain its constant assault on the right to counsel?

These ads are dangerous, toxic, and horrifying. And they aren’t isolated incidents. Across the board, Republican leadership seems to agree that attacking former defense attorneys for fulfilling Sixth Amendment duties is perfectly acceptable politics. Republicans who specialize in accusing Democrats of pushing unconstitutional measures have remained silent about the assault on the right to counsel. Remember that the next time they rant and rave about Obama’s purported disregard for the Constitution. Democrats may disagree with certain Supreme Court decisions and stretch constitutional lawmaking to its limits—but Republicans have decided to attack the Constitution directly. The Party of Lincoln has finally abandoned the principles that Lincoln cherished the most.