There are conservatives who lie and there are liberals who lie. Neither blue nor red has a monopoly on truth-tellers.
However, unless one denies that there are distinctive values on the right and on the left a proposition that no serious liberal or conservative would deny how much truth is valued may be different for the right and the left.
In the hierarchy of leftist (as opposed to traditional liberal) values, truth is below other values, such as equality, opposition to war, the promotion of secularism and a number of other highly regarded values on the left.
This does not mean that the number of truth-tellers among individuals on the left is necessarily smaller than the number of individual truth-tellers on the right. It means that truth-telling is not high on the left's list of values.
Since this is, obviously, a generalization, and a negative one at that, anyone who makes this generalization is obligated to provide arguments and examples.
The first example is what is known as political correctness. Leftist denial of what is true is so widespread that we have a term for it, political correctness. There is no comparable right-wing political correctness, i.e., denying truths so as not to offend right-wing values or certain groups.
For example, among many on the left, especially academics, it has been almost impossible for decades to tell the truth about the innate differences between men and women because of the leftist dogma of innate similarities between the sexes. So deep is the left's hostility to truth regarding the sexes that a president of Harvard University was forced from office after suggesting that men's and women's brains process math and some science differently.
Similarly, many leftist professors at Duke University used the false rape charges against three white lacrosse players to reinforce the left-wing belief (itself not true) that America is racist. The truth was not nearly as important to them as proving how racist whites are.
Textbooks. A prime example of the left's view of truth is its changing the goal of high school American history textbooks from telling truth to promoting self-esteem among minority and female students by depicting more women and more non-whites in American history textbooks.
"Bush is a liar." Currently, the most widely repeated lie of the left is that President George W. Bush lied about Saddam Hussein having weapons of mass destruction. It is repeated so often ("Bush lied, people died") that many Americans now believe this. But it is not true. There were valid reasons for anyone to believe that Saddam Hussein had WMD. Saddam had used them in the past; he refused to allow unfettered inspections; he was the major foreign sponsor of Palestinian terror; and most important, virtually all Western intelligence agencies believed Saddam had WMD.
Nor did President Bush lie, as the left frequently charges, about Saddam seeking uranium in the African nation of Niger. The president said in his 2003 State of the Union address that "The British Government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa." That was exactly what British intelligence reported, and the British intelligence is now widely believed to have been accurate. It is a left-wing lie that President Bush lied when he spoke those now famous 16 words.
Callling liberals "unpatriotic." Another lie of the left is that Republicans and conservatives regularly label opponents of the war in Iraq "unpatriotic." Thus, during the CNN/YouTube debate, Sen. Hillary Clinton claimed that "I asked the Pentagon a simple question: 'Have you prepared for withdrawing our troops?' In response, I got a letter accusing me of being unpatriotic." That is and this is not said easily a lie. Anyone who reads the Defense Department response to Sen. Clinton will see that what she claims is entirely untrue. Her patriotism, or lack of it, was not even hinted at. Moreover, it is rare almost to the point of nonexistent for mainstream Republicans or conservatives to call any liberal critic of the Bush administration "unpatriotic."
The homeless, heterosexual AIDS and rape. For years, mainstream liberal news media purveyed false information supplied by Mitch Snyder, the major liberal activist on behalf of the homeless. Likewise, we were told by gay and AIDS activist groups that AIDS "doesn't discriminate," meaning that heterosexuals in America were as likely to contract the HIV virus as homosexuals. It was never true in America (Africa may be another story for other reasons). Feminist groups have offered statistics on rape and sexual violence that are patently false.
Few liberal activist groups tell the truth. Not because their members are liars in private life they may well be as honest as anyone else but because whatever the left advocates it deems more important than truth.
This does not mean the right is always honest. For example, conservatives who say that "pornography causes rape" are doing what the left does putting their agenda, in this case a loathing of pornography, above truth-telling. I have seen no credible statistics linking the proliferation of pornography with increased rape.
But when the left ceaselessly repeats the mantra "Bush lied," it may simply be projecting onto George W. Bush what comes quite naturally to the left when it offers false Iraqi death statistics, false homeless data, false rape statistics, false secondhand smoke statistics, false claims about the percentage of gays in the population, and false claims of just about everything else the left cares about.