Senate Judiciary Chairman Patrick J. Leahy and ranking Republican Charles E. Grassley sparred at the opening of Monday’s hearing on immigration changes over how the Boston Marathon bombings might affect the legislation.

“Let me point out one thing that has troubled me a great deal,” Leahy said. “Last week, opponents of comprehensive immigration reform began to exploit the Boston Marathon bombing. I’m a New Englander; I’ve spent a lot of time in Boston growing up and still do today. [I have] friends and relatives there. I urge restraint in that regard.

The Vermont Democrat added, “Let no one be so cruel as to try to use the heinous act of these two young men last week to derail the dreams and futures of millions of hardworking people. The bill before us would serve to strengthen our national security by allowing us to focus our border security and enforcement effort against those that do us harm.”

Grassley, who connected the two issues at the April 19 immigration hearing, reacted sharply Monday by saying he just wants a thorough and deliberative process. The Iowa Republican also noted that when Leahy and Democrats proposed gun control legislation, Republicans did not accuse Democrats of exploiting the tragedy of Newtown, Conn., where 20 schoolchildren and seven adults were killed in December.

“I want you to take note of the fact that when you proposed gun legislation, I didn’t accuse you of using the [Newtown] killings as an excuse,” Grassley said.

Grassley also cited the fertilizer plant explosion in West, Texas, that killed 14 people, noting that he also doesn’t see criticism of people “taking advantage of that tragedy to warn about more government action to make sure that fertilizer factories are safe.”

“I think we are taking advantage of an opportunity when once in 25 years we deal with immigration to make sure that every base is covered,” Grassley said.

Leahy also announced that Homeland Security Secretary Janet Napolitano would appear before the committee Tuesday after canceling her appearance last week to deal with the ongoing search for the Boston bombing suspects.

“Secretary Napolitano was scheduled to appear then, but I think everybody understands that [given] what happened in Massachusetts she could not be here,” Leahy said. “She will be before the committee tomorrow morning.”

As lawmakers and the public learned last week that the two men suspected of masterminding the Boston bombings were immigrants with roots in Chechnya, some in Congress said the Senate should slow its rush to pass the bill until it learned more about the suspects’ immigration history.

“When we find out, it will help shed light on the weaknesses of our system, how an individual evades such authority and plans attacks on our soil,” Grassley said April 19.

Speaking on ABC’s “This Week” on Sunday morning, Sen. Dan Coats, R-Ind., said he agreed with Grassley. “We have a broken system; it needs to be reformed,” he said. “But I’m afraid we’ll rush to some judgments relative to immigration and how it’s processed. So let’s do it in a rational way rather than an emotional way.”

On Monday, Sen. Charles E. Schumer, D-N.Y., another member of the immigration group, denounced those who want to use the bombing as an “excuse” to delay immigration legislation, drawing an impassioned response from Grassley, who said, “I didn’t say that.”

Schumer said the bill includes added border security measures, more money for the border fence, biometric indicators for new immigrants and an improved entry and exit system at the nation’s airports and seaports, all designed to prevent those who would commit terrorist acts from entering the country.

“If there are things that come up as a result of what happened in Boston that require improvements, let’s add them to the bill,” he said. “But certainly our bill tightens up things in a way that would make a Boston less likely.”

The immigration ramifications of the Boston bombings are unlikely to change many minds on Capitol Hill, where momentum for an immigration overhaul has been building since the 2012 elections, when Hispanic voters helped propel President Barack Obama back to office.

But Grassley’s statement suggests a new line of attack that opponents of the immigration bill could attempt to use to unravel the bipartisan effort.

Republicans supporting the overhaul took Grassley to task last week for his statements.

Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., who are members of a bipartisan group of eight senators working on an immigration bill, released a joint statement urging restraint.

“In the wake of this week’s terrorist attack in Boston, some have already suggested that the circumstances of this terrible tragedy are justification for delaying or stopping entirely the effort for comprehensive immigration reform,” they said. “In fact, the opposite is true: Immigration reform will strengthen our nation’s security by helping us identify exactly who has entered our country and who has left, a basic function of government that our broken immigration system is incapable of accomplishing today.”

In response to Leahy’s comments, Sen. Marco Rubio, R-Fla., one of the bipartisan group who drafted the bill, said he believes there is a connection between the Boston bombing and the immigration proposal because that measure is being pursued as an effort to help make the nation safer.

“I disagree with those who say that the terrorist attack in Boston has no bearing on the immigration debate,” Rubio said. “Any immigration reform we pursue should make our country safer and more secure. If there are flaws in our immigration system that were exposed by the attack in Boston, any immigration reform passed by Congress this year should address those flaws. Congress needs time to conduct more hearings and investigate how our immigration and national security systems could be improved going forward.”

“The attack reinforces why immigration reform should be a lengthy, open and transparent process, so that we can ask and answer important questions surrounding every facet of the bill,” Rubio said. “But we still have a broken system that needs to be fixed.”

On Monday, Leahy urged his colleagues to work together during the amendment process to improve the bill rather than attempt to kill it.

“Too often in the recent past this committee has broken along partisan lines on compelling issues,” he said, referring to the renewal of the Violence Against Women Act, which ultimately passed the full Senate on a bipartisan vote, and the recent votes on gun legislation.

“I do not want to see comprehensive immigration reform fall victim to entrenched, partisan opposition,” he said.