New York Mayor Michael Bloomberg personally wrote Wednesday to hundreds of Big Apple donors, asking them to stop funding four Democratic senators who opposed tougher gun laws in Congress.

The move by the billionaire mayor, who has long campaigned for tighter gun controls, comes as the nation prepares to mark on Friday the six-month anniversary of the deadly shooting at an elementary school in Connecticut.

In his letter, Bloomberg said Senators Mark Begich of Alaska, Heidi Heitkamp of North Dakota, Mark Pryor of Arkansas and Max Baucus of Montana had "sided with a gun lobby increasingly out of touch with Americans' priorities."

"The next time these four senators want you to support them with donations to their campaigns, tell them you cannot," said the 71-year-old Bloomberg.

Baucus has already said he is not planning to run for re-election in 2014.

Since the December 14 massacre in Newtown, in which 20 children and six adults were shot dead by a heavily-armed 20-year-old who later killed himself, a few US states have passed tougher gun laws, notably making background checks mandatory.

But in April, the Senate failed to pass a watered-down package of gun reform measures that would have expanded background checks for gun buyers, a vote President Barack Obama called "shameful."

Republicans argued that the bill infringed on the constitutional right to bear arms, but Obama said it did no such thing and merely tried to stop criminals and the mentally ill from getting firearms.

Some conservative Democrats also opposed the bill, perhaps fearing a backlash ahead of their re-election bids.

To mark the six-month anniversary of the Newtown shooting, Mayors Against Illegal Guns -- a group co-founded by Bloomberg -- has announced a 25-state tour over 100 days.

Gun violence survivors and supporters will read the names of the more than 5,000 Americans killed by firearms since the Newtown shooting, in front of the local offices of members of Congress.

Bloomberg has spent millions of dollars of his own money on political ads via the advocacy group, appealing for reform and trying, often with success, to prevent the re-election of candidates backed by the powerful US gun lobby.