To understand how Democrats plan to hold the Senate, consider President Obama's claim about the Affordable Care Act: "If you like your plan, you can keep your plan." Why did he so often say something that wasn't true?
In 2009 and early 2010, he made this false promise to pass the Affordable Care Act. It's unlikely every congressional Democrat would have voted for it if their constituents knew the truth. And from March 2010 through November 2012, he said it to get re-elected. If voters knew what was coming, more Democrats would have lost, and perhaps even the president himself.
The lesson many Democrats seem to have taken away is that the benefits of misleading voters outweigh the downsides. At least that view seems to be guiding Democrats in the battle for the Senate, especially the work of Majority Leader Harry Reid's Senate Majority PAC.
Consider Republican Congressman Tom Cotton, who is challenging Arkansas Sen. Mark Pryor. A Senate Majority PAC ad claimed Mr. Cotton "got paid handsomely working for insurance companies." Actually, the management consultants McKinsey & Company employed Mr. Cotton. The Afghanistan and Iraq veteran's only work involving insurance was helping the Federal Housing Administration to improve services, including insurance, for companies that finance apartment construction. His annual salary was $85,000. But Democrats are determined to portray Mr. Cotton as a rich tool of insurance companies, no matter how many lies are involved.
Senate Majority PAC also ran an ad implying that Louisiana Republican Senate hopeful Bill Cassidy was part of "the fight to let flood insurance premiums soar, helping the insurance companies and cut off hurricane relief for Louisiana families." In reality, Rep. Cassidy and several other Republican congressmen from coastal regions proposed an amendment that kept flood-insurance premiums from rising. The idea that a congressman from the state hit by Katrina would "cut off hurricane relief" is absurd.
Campaigns often make shaky claims, but these Democratic ads are flat-out falsehoods. Republicans have an opportunity to counterpunch powerfully. Many Republican candidates are doing so, diminishing the credibility of Democrats while offering a governing agenda for the middle class. GOP candidates must be disciplined and have the resources to fight back.
Democrats are hoping to repeat 2010 and 2012, when opportunities for GOP pickups were squandered by weak Republican candidates, such as Todd Akin in Missouri and Richard Mourdock in Indiana. Democrats and their super PACs are spending money in 2014 Republican primaries to defeat the strongest candidate in the field.
For example, the Put Alaska First PAC boosting Sen. Mark Begich's re-election received 82% of its money from Harry Reid's Senate Majority PAC. In turn, Put Alaska First attacked Dan Sullivan, the GOP front-runner, for owning a home in Maryland, suggesting he was not an Alaskan.
American Crossroads (a group I helped organize) replied with an ad featuring former Secretary of State Condoleezza Rice. She explained Mr. Sullivan's family had a home in suburban D.C. when, as a decorated Marine, he was stationed at the White House and then, after two tours in the Middle East, returned to serve as assistant secretary of state. The ad also reinforced his record as Alaska's attorney general and Natural Resources Commissioner.
In North Carolina, Senate Majority PAC wants to keep North Carolina House Speaker Thom Tillis from winning 40% in the May 6 Republican primary and force him into an expensive runoff. It has unleashed a $973,000 attack on Mr. Tillis over the 2012 resignations of two of his legislative aides caught having affairs. This is more than all seven of Mr. Tillis's GOP primary opponents are spending on ads, combined.
Left unanswered, these Democratic attacks might work. But if addressed effectively, they are unlikely to prevail. GOP candidates should ask Republican primary voters if they really want Harry Reid, smear artist extraordinaire, picking their nominees.
Democrats may also hope their attacks cause voters to stay home. This worked in 2012, when months spent bashing Mitt Romney's career at Bain and his wealth caused the number of presidential voters to decline for the first time in 16 years. Many who didn't vote were disposed to Mr. Romney.
The Romney campaign didn't respond, feeling that if they were responding they were losing. But responding often means demolishing your opponent's credibility while establishing yours by offering voters better ideas and a brighter future. Candidates who identify with the concerns of voters while Democrats try to alienate them may help the GOP remove Mr. Reid as Senate majority leader this fall.
A version of this article appeared April 24, 2014, in the U.S. edition of The Wall Street Journal, with the headline Harry Reid Tries To Pick GOP Candidates and online at WSJ.com.