What is it about presidents' second terms that makes them seem so scandal-ridden? Simple: The iron law of longevity. All governments make mistakes, and all governments try to hide those mistakes. The longer an administration is in office, the more errors it makes, and the harder they are to conceal.

Just ask Richard M. Nixon, Ronald Reagan, Bill Clinton or George W. Bush, all of whom spent much of their second terms playing defense.

The rule caught up with Barack Obama last week as he wrestled clumsily with three controversies: the Internal Revenue Service's treatment of tea party groups, the Benghazi killings and the Justice Department's seizure of Associated Press telephone records.

The president's Republican critics reached for historical comparisons: It's another Watergate, said some. Another Iran-Contra, said others.

So far, though, the three imbroglios don't add up to another Watergate; not even close. But there are enough unanswered questions to keep any administration tied up for months in hearings, and that's exactly what's about to happen.

The IRS scandal is the most straightforward: A unit of the tax agency applied political criteria to its scrutiny of applications for tax-exempt status. Despite the initial portrayal of a rogue operation confined to Cincinnati, IRS officials in Washington knew about the problem and failed to fix it. At least one appears to have misled Congress by suggesting that tea party complaints were unfounded.

Last week, Obama condemned the IRS conduct as "intolerable and inexcusable," and he fired the agency's acting director.

But every customer of the IRS should want an independent investigation to determine whether higher-ups encouraged the Cincinnati cabal.

Benghazi is the most tangled issue, and the most partisan. A State Department review board has concluded that security for Ambassador J. Christopher Stevens' fatal visit to the Libyan city was inexcusably weak; then-Secretary of State Hillary Rodham Clinton accepted responsibility.

But the political side of the Benghazi "scandal," which still needs to be in quotes, has focused on other targets. Republicans charge that Obama lied about the attacks, portraying them as spontaneous to avoid weakening his election-year claim that he had put al-Qaida on the path to defeat.

It's true that Obama was slow to blame terrorists for the killing. "We don't have all the information yet," he said on Sept. 25. But by then, other officials had already told Congress that al-Qaida affiliates were involved. If the White House was trying to mount a cover-up, it apparently forgot to tell the rest of the administration.

It's also true that Obama aides presided over an internal debate over what information would be in official talking points. But when the White House finally released emails from that wrangle, they mostly revealed a bureaucratic fight between the CIA, which wanted to trumpet its warnings about Libya, and the State Department, which didn't want to expose its failings.

The Benghazi talking points look mostly like a partisan sideshow, too complicated and murky to engage most voters.

The third controversy, over the Justice Department's secret decision to seize telephone records of dozens of reporters and editors at the Associated Press, is a different kind of scandal. Republicans have been careful about this one because many have long demanded that the Obama administration get tough on leaks of classified information.

But it still fits into the GOP's critique of Obama as imperious and authoritarian, a onetime civil libertarian who has grown fond of executive power in office.

If Obama is smart and lucky, all three controversies will gradually fade, assuming no more wrongdoing comes to light. If his Republican critics hound the White House on claims that don't pan out, they'll be vulnerable to charges they're wasting time on partisan squabbles.

But a season of scandal still comes with a cost. If Congress spends much of its time on investigations (and one-third of all House committees have announced they plan to do just that), it will have fewer hours to work on other issues. If the White House must focus on defense, that saps its energy as well.

Any second-term president has limited time to win legislative battles. Obama's clock is already ticking; his agenda is already in trouble. If the remainder of 2013 is dominated by inquests that widen the partisan divide, the chances for bipartisan deal-making - especially a grand bargain on taxes and spending - will wane even further.

Readers can email Doyle McManus at doyle.mcmanus@latimes.com.