It does not appear that Portia would have tried to stop her husband from participating in the assassination of Julius Caesar. In fact, she becomes a sort of co-conspirator. In Act 2, Scene 1, she succeeds in getting her husband to promise to tell her why he has been acting so strangely and why he has been surreptitiously consulting with so many men. Brutus cannot tell her at that point because someone is knocking, but he says:
Hark, hark, one knocks. Portia, go in awhile,
And by and by thy bosom shall partake
The secrets of my heart.
All my engagements I will construe to thee,
All the charactery of my sad brows.
Leave me with haste.
He has promised to tell her all the secrets of his heart and all his plans. She has been successful in persuading him that he owes her his complete trust and confidence because of their relationship as man and wife. (Many women feel that complete openness is an essential element of marriage.) Then in Act 2, Scene 4, we see that Portia now knows everything. Shakespeare evidently did not feel it necessary to have a scene in which Brutus explained his intentions, since the audience already knows everything that Brutus could have told his wife, including his misgivings. In Act 2, Scene 4, Portia evidently wants the assassination to be successful, and she has not apparently tried to talk her husband out of going through with it. After hearing from Brutus how far things had progressed and how many others were involved, she must have realized that, to use Caesar's expression, the die was cast. There was no turning back. If the conspirators were to call off the assassination attempt for any reason, they would be exposed for plotting it; and one by one they would all be killed. Since Portia knows the men are going to try to kill Caesar that day, she is, in effect, a co-conspirator with her husband. She reveals her guilty knowledge as well as her fears in talking partly to the boy Lucius and partly to herself.
I must go in. Ay me, how weak a thing
The heart of woman is! O Brutus,
The heavens speed thee in thine enterprise!
Sure, the boy heard me. Brutus hath a suit
That Caesar will not grant. O, I grow faint.
Run, Lucius, and commend me to my lord;
Say I am merry. Come to me again,
And bring me word what he doth say to thee.
When Portia says, "The heavens speed thee in thine enterprise," she shows she knows exactly what is going to happen. Then she is afraid the boy may have overheard these words. So she tells him: "Brutus hath a suit that Caesar will not grant." Portia feels helpless. She is confined to her home. She can do nothing but wait and wonder what is happening. Perhaps she now regrets that she persuaded her husband to share all his secrets with her. (Perhaps complete openness is not the ideal component of a marriage?) If the plot fails she will lose everything. We see in Act 2, Scene 3 that there is a possibility that the assassination attempt might be thwarted. For instance, Artemidorus has prepared a warning letter for Caesar, naming names.
“Caesar, beware of Brutus; take heed of Cassius;
come not near Casca; have an eye to Cinna; trust not
Trebonius; mark it well Metellus Cimber; Decius Brutus
loves thee not; thou hast wronged Caius Ligarius. There is
but one mind in all these men, and it is bent against Caesar.
If thou beest not immortal, look about you. Security gives
way to conspiracy. The mighty gods defend thee!
Thy lover, Artemidorus.”
But Caesar refuses to heed Artemidorus's urging to read the paper immediately and goes to his death.
It is clearly evident in Julius Caesar that Portia is devoted to her husband and has great loyalty and love for him. Were she able to prevent his part in the assassination, it would certainly have been in character for her to have made strong attempts to arrest Brutus from joining the conspirators.
In Act II, Scene 1, Portia, who has long been the confidant of her husband, awakens and finds Brutus has stolen "out of his wholesome bed" to walk in the dangerous night air. She entreats him to "make me acquainted with your cause of grief" (2.1.264). Brutus replies that he does not feel well, but Portia knows that there is more to his "sickness":
No, my Brutus
You have some sick offense within your mind,
Which by the right and virtue of my place
I ought to know of, and upon my knees
I charm you, by my once commended beauty
By all your vows of love, and that great vow...
That you unfold to me, your self, your half,
Why you are heavy [sorrowful, anxious] and what men tonight
Have had resort to you.... (2.1.275-285)
Portia even kneels before her husband in her entreaty; however, Brutus refuses to confide in her, even when she stabs herself with her knife in her thigh in order to prove that she is the daughter of a noble man and brave.
By the fact that she has sought Brutus in the night, beseeching him to reveal his cause of anxiety, and made every effort to win his confidence, Portia would in all probability go to any lengths to save Brutus from acting ignobly by participating in Caesar's assassination. That she is loyal and brave is also later demonstrated in Act IV, Scene 3, when she learns that Brutus has been forced to flee and his life is threatened, she eats hot coals and kills herself much in the manner that losing warriors in battle kill themselves rather than suffer dishonor.
On an interesting historical note, critics have observed that Portia's impassioned remarks about being "stronger" and more possessive of "constancy" than most women, recalls Queen Elizabeth I's famous comment,
"I know I have the body but of a weak and feeble woman; but I have the heart and stomach of a king" ("Speech to the Troops at Tilbury", 1588)
Thus, Shakespeare affirms the capability of a strong woman's being able to act as a brave man would, thus reinforcing the idea that Portia would make every effort to arrest her husband's dangerous actions.